This invention relates to an AM stereo broadcast system for the transmission of two signals on a single carrier and more particularly to an improved system for transmitting and receiving fully compatible AM stereo signals on the AM broadcast band on monaural and stereo receivers without substantial distortion.
Several systems for transmitting and receiving AM stereo signals are known in the art. The simplest system is probably an unmodified quadrature signal which transmits two signals A, and B, e.g., left (L) and right (R), on two carriers which are identical in frequency but are in phase quadrature. This system is similar to the system used to transmit the two color signals on one carrier in the NTSC standard for U.S. color television transmission. On existing monaural receivers, using signal current rectifiers to derive the audio signal, however, there is double frequency distortion which is proportional to the amount of the stereo difference (L-R) signal. The distortion arises from the fact that this signal consists basically of the following: ##EQU1## where the term under the radical is the amplitude and where .phi.=tan.sup.-1 (L-R)/(1+L+R). The monaural receiver, however, requires that the amplitude of the received signal be substantially the carrier plus the audio, or (1+L+R). The (L-R) term thus represents distortion, and,--since it is a squared term,--double frequency distortion. The .phi. term represents phase modulation and produces no output from a conventional envelope detector in a monaural receiver when there is no appreciable amplitude or phase distortion present on the signal in the entire system.
Still another prior system employs the technique of transmitting a single carrier, which is amplitude modulated with (L+R) information and frequency modulated with (L-R). The complex spectrum of the transmitted signal may give rise to undesirable distortion in both monaural and stereo receivers if any frequency or phase distortion is present in the received signal. When the (L-R) signal contains low frequency components, the radiated spectrum may contain many sideband frequencies which are subject to distortion in phase and amplitude which, in turn, produces spurious conversion of FM components to amplitude modulation.
Yet another system transmits sum and difference signals in quadrature, but distorts the (L+R) component to correct the amplitude of the envelope and make it compatible. This is done by changing the in-phase component from (1+L+R) to ##EQU2## and keeping the magnitude of the quadrature component unchanged. The phase or stereo information is thus distorted and the number of significant sidebands is increased, increasing the potential distortion on both monophonic and stereo receivers.